1. Field of the Invention
This invention relates to an aluminum indium gallium nitride (AlxInyGa1-x-yN: 0≦x≦1, 0≦y≦1, 0<x+y≦1) mixture crystal substrate for producing ultraviolet, blue light emitting diodes (LEDs) and ultraviolet, blue light laser diodes (LDs) composed of groups 3-5 nitride type semiconductors, a method of growing an aluminum indium gallium nitride (AlxInyGa1-x-yN: 0≦x≦1, 0≦y≦1, 0<x+y≦1) mixture crystal substrate, and a method of producing an aluminum indium gallium nitride (AlxInyGa1-x-yN: 0≦x≦1, 0≦y≦1, 0<x+y≦1) substrate.
This application claims the priority of Japanese Patent Applications No. 2001-311018 filed on Oct. 9, 2001 and No. 2002-269387 filed on Sep. 17, 2002, which are incorporated herein by reference.
A substrate means a thick freestanding base plate on which thin films are grown. A substrate should not be confused with thin films of similar components. The scope of component ratios of the AlxInyGa1-x-yN mixture crystals of the present invention is defined by mixture parameters x and y. Designated ranges are 0≦x≦1, 0≦y≦1 and 0<x+y≦1. When x=0, AlxInyGa1-x-yN means InyGa1-xN (0<y≦1; abbr. InGaN). No prior art of making InGaN substrates was found. When y=0, AlxInyGa1-x-yN means AlxGa1-x-yN (0<x≦1; abbr. AlGaN). The inventors have found no prior art of making AlGaN substrate yet. The third inequality 0<x+y≦1 forbids the case that x and y simultaneously take 0 value (x=0 and y=0). Thus GaN is excluded from the collective expression of AlxInyGa1-x-yN (0≦x≦1, 0≦y≦1, 0<x+y≦1) of the present invention. GaN is excluded from the scope of the present invention. When x and y are not zero, AlxInyGa1-x-yN is sometimes abbreviated to AlInGaN. No prior art of producing AlInGaN substrates has been found.
The bandgap energy Eg of a semiconductor is proportional to an inverse of the wavelength λ of light which is absorbed into or emitted from the semiconductor. A simple inverse relation λ(nm)=1239.8/Eg(eV) holds between Eg and λ. Gallium nitride has a bandgap of 3.2 eV. Emission or absorption light wavelength is 387 nm for GaN. Aluminum nitride AlN has a high bandgap energy Eg=6.2 eV. Emission or absorption light wavelength is 200 nm for AlN. Indium nitride InN has a low bandgap energy of Eg=0.9 eV. Emission or absorption light wavelength is 1378 nm for InN. If light emitting devices having a light receiving layer composed of aluminum indium gallium nitride AlxInyGa1-x-yN were produced, the devices could produce various light having a wide range wavelength between about 200 nm and about 1300 nm. However, such devices relying upon nitride semiconductors have not been made yet. The reason why such nitride semiconductor devices have not been made is that no pertinent substrates are available. Light emitting devices (light emitting diodes or laser diodes) are made by preparing a substrate wafer, growing several semiconductor thin films on the substrate, etching some regions of some films, doping the films with dopants and forming electrodes. The substrate is a starting material. The lattice constant of aluminum nitride (AlN) is 0.3112 nm. The lattice constant of gallium nitride (GaN) is 0.3189 nm. The lattice constant of indium nitride (InN) is 0.3545 nm. Sapphire (α-Al2O3) which has been used as a substrate of InGaN-type blue ray LEDs is not a promising candidate for AlInGaN devices. Sapphire is insulating and uncleavable. Sapphire is not a promising candidate for producing nitride semiconductor devices thereon. Nitride semiconductor substrates would be preferable to sapphire as substrates for making nitride light emitting devices.
Among simple nitride compounds GaN, InN and AlN, only GaN substrates can now be produced by the applicant's endeavors. But nobody has succeeded in producing InN substrates or AlN substrates of good quality. Neither InN substrates nor AlN substrates are available at present. If indium nitride InN films were epitaxially grown on an obtainable GaN substrate, large lattice misfit would induce a lot of defects and strong stress in the InN films. If aluminum nitride AlN films were grown on an obtainable GaN substrate, the AlN film would be plagued with many defects. Thus GaN wafer is not a pertinent candidate as a substrate of making AlxInyGa1-x-yN films.
The most suitable substrate for making AlxInyGa1-x-yN (0≦x≦1, 0≦y≦1, 0<x+y ≦1) films is an AlxInyGa1-x-yN: 0≦x≦1, 0≦y≦1, 0<x+y≦1 substrate having the same values of x and y as the epitaxial films. If an AlxInyGa1-x-yN (0≦x≦1, 0≦y≦1, 0<x+y≦1) were obtainable, AlxInyGa1-x-yN films would be grown homoepitaxially on the AlxInyGa1-x-yN substrate.
The purpose of the present invention is to provide an AlxInyGa1-x-yN: 0≦x≦1, 0≦y ≦1, 0<x+y≦1 mixture crystal substrate and a method of producing an AlxInyGa1-x-yN: 0≦x≦1, 0≦y≦1, 0<x+y≦1 mixture crystal substrate.
2. Description of Related Art
No prior art of AlxInyGa1-x-yN: 0≦x≦1, 0≦y≦1, 0<x+y≦1 mixture crystal substrates has been found. There are several references about the method of making GaN substrates contrived by the inventors of the present invention. Although AlxInyGa1-x-yN: 0≦x≦1, 0≦y≦1, 0≦y≦1, 0<x+y≦1 does not include GaN, some GaN-related documents are now described instead of AlxInyGa1-x-yN substrates.
The inventors of the present invention contrived a GaAs-based epitaxial lateral overgrowth method (ELO) for making low-dislocation GaN crystals by preparing a GaAs substrate, making an ELO mask having many small regularly-populated windows on the GaAs substrate, and growing GaN films by a vapor phase growing method on the ELO-masked GaAs substrate. The inventors had filed a series of patent applications based on the GaAs-based ELO methods for making GaN crystal bulks.    {circle around (1)} Japanese Patent Application No. 9-298300    {circle around (2)} Japanese Patent Application No. 10-9008    {circle around (3)} Japanese Patent Application No. 10-102546    ({circle around (1)}, {circle around (2)} and {circle around (3)}) have been combined into a PCT application of WO 99/23693.)    {circle around (4)} Japanese Patent Laying Open No. 2000-012900    {circle around (5)} Japanese Patent Laying Open No. 2000-022212
The ELO method makes a thin GaN film on an undersubstrate by forming a mask layer (SiN or SiO2) on the undersubstrate, etching small dots aligning in a small regular pattern of an order of micrometers on the mask, forming regularly aligning small windows, growing a GaN layer on the exposed undersubstrate in the windows in vapor phase, making dislocations running upward in a vertical direction at an early stage, turning the dislocations in horizontal directions, inducing collisions of dislocations and reducing the dislocations by the collisions. The ELO method has an advantage of reducing the dislocations by the twice changes to the extending direction of the dislocations. The ELO method enabled the inventors to make a thick (about 100 μm) GaN single crystal.    {circle around (6)} Japanese Patent Laying Open No. 2001-102307 (Japanese Patent Application No. 11-273882)
GaN facet growth was proposed in the document {circle around (6)} by the same inventors as the present invention. All the known GaN growing methods had been C-plane growth maintaining a smooth, flat C-plane as a surface of c-axis growing GaN. {circle around (6)} denied the conventional C-plane growth and advocated a new idea of facet growth which grows GaN, makes facets on a growing GaN, forms pits of the facets, maintains the facets and pits without burying pits, pulls dislocations into the facets, attracts the dislocations into the pits, reduces dislocations outside of the pit bottoms and obtains low dislocation density GaN crystals.
FIG. 1 to FIG. 3 show our previous facet growth of GaN. FIG. 1 is an enlarged view of a facet pit on a surface of a GaN crystal during the facet growth. In FIGS. 1(a) and 1(b), a GaN crystal 2 is growing in a c-axis direction in average. The GaN crystal 2 has a C-plane top surface 7. Crystallographical planes inclining to the C-plane are called facets 6. The facet growth forms facets 6 and maintains the facets 6 without burying. In the example of FIG. 1, six facets 6 appear and form a polygonal reverse cone pit 4 dug on the C-plane surface 7. The pits 4 built by the facets 6 are hexagonal cones or dodecagonal cones. Hexagonal pits 4 are formed by the six-fold rotation symmetric facets 6 of either {11-2 m} or {1-10 m} (m: integer). Dodecagonal pits are composed of {11-2 m} and {1-10 m} (m: integer). Although FIG. 1(a) and FIG. 1(b) show the hexagonal pit, dodecagonal pits appear prevalently.
To form facet pits, to maintain pits and not to bury pits are the gist of the facet growth. The facet 6 moves in a direction normal to the facet 6. A dislocation extends along a growing direction. A dislocation extending along a c-axis and attaining the facet 6 turns its extending direction in a horizontal direction parallel to the facet 6 and reaches a crossing line 8. The crossing lines 8 include many dislocations. As the top surface 7 moves upward, the dislocations gathering on the crossing lines 8 make defect gathering planes which meet with each other at 60 degrees. Planar defect assemblies 10 are formed below the crossing lines 8 (FIG. 1(b)). The planar defect assemblies 10 are in a stable state. Some dislocations attaining to the crossing lines 8 turn their extending directions again inward, move inward along the rising slanting crossing lines 8 and fall into a multiple point D (FIG. 2) at a pit bottom. Dislocations substantially run inward in the horizontal directions. A linear defect assembly 11 is formed along the multiple point D at the pit bottom. The linear defect assembly 11 is less stable than the planar defect assemblies 10. FIG. 3 (1) demonstrates the function of the facet pit gathering dislocations. A C-plane surface 17 has a facet pit 14 composed of facets 16. When the C-plane surface 17 and the facets 16 rise, dislocations are attracted into the facet pit 14, pulled into the bottom of the pit 14 and captivated into a bundle 15 of dislocations at the bottom of the pit 14. But the bundle 15 of dislocations is not everlasting. The dislocations once gathering to the bottom again disperse outward. Then dislocations are released. FIG. 3 (2) demonstrates the dispersion of dislocations 15 in radial directions into surrounding portions 12. The dislocation density rises again in the surroundings of the pits 14. {circle around (6)} was still incompetent to make thick low dislocation AlInGaN crystals.